The present invention relates broadly to a color monitor apparatus, and in particular to an improved resolution delta gun color monitor apparatus.
The man-machine interface in air traffic control systems is still primarily the cathode ray tube (CRT) despite prophacies of its demise for at least the last 20 to 25 years. Of course as time has passed, the demands of ever increasing data loads has required the development of high resolution CRT's to meet the system requirements.
In an effort to ease some of the burden on the air traffic controller, color displays are beginning to be employed in some of the latest air traffic control systems. In these systems, color is employed to differentiate various types of data but does not reduce the amount of information which is actually required to be displayed at a given time by the display.
At the present time, the color displays that are currently in use, are in some military systems. The largest displays that are used in these present systems, employ a 19 inch color shadow mask CRT.
During the last year or so, an air traffic control system has been under development which is to utilize a 25 inch delta gun shadow mask CRT. At the present time, there are only two suppliers of high resolution 25 inch color CRT's of this type being made in the free world (Mitsubishi and Matsushita). When this development was started, insufficient data was available to indicate Which of these CRT's was better for this system.
In the last few months, competing monitor vendors have supplied monitors for evaluation. One vendor supplied several prototypes that was specified for this system. While one of the CRT vendor's tubes were found to have somewhat better resolution performance than the other, both fell short at the edges of the raster and particularly in the corners.
One of the available CRT's was investigated by one of the competing display vendors in conjunction with the CRT manufacturer after evaluation of their monitor indicated their monitors fell short of the desired resolution performance, particularly near the edges and corners of the display. Perhaps the greatest single problem identified in their reports was the spread in the best center focus voltage for each of the three electron guns employed for colors of the delta gun, shadow mask color CRT. The cause of this focus spread was identified as being caused by the unavoidable manufacturing tolerances in the spacings and alignment of the parts and components that make up each of the three separate guns.
Extensive tests have been performed on the resolution performance of the CRT in the first engineering prototype monitor from a monitor vendor employing the second of the two available CRT manufacturers tubes. This tube also exhibited similar problems with the spread of the focus voltage that was required for maximum resolution of each of the three electron guns.
The accepted production tolerance limits for the magnitude of the difference in the best focus potentials for both of the available high resolution CRT manufacturers tubes exceeded the dynamic focus potential that was recommended by each of the CRT manufacturers as indicated on their respective CRT data sheets.
Careful evaluation of the test data obtained from each of the two available CRT manufacturer tubes showed the performance of any one of the electron guns employed alone in a tube exhibited considerably better edge and corner resolution performance than that typically achieved by the tube as a whole since all three electron guns are tied to common focus potential.
It has been common industry practice to tie all three electron guns to a common focus potential so as to simplify both the CRT construction as well as the associated monitor circuitry. This has been permissible in the past because the delta gun color CRT's were originally developed for and used in the entertainment industry for home television sets. It has only been recently that color CRT's are being employed in applications demanding much higher resolution such as the Cad-Com industry and now the air traffic control applications.
The state of the art of color monitor apparatus is well represented and alleviated to some degree by the prior art apparatus and approaches Which are contained in the following U.S. patents:
U.S. Pat. No. 3,887,837 issued to Nakayama et al on June 3, 1975;
U.S. Pat. No. 4,546,291 issued to Morioka et al on Oct. 8, 1985; and
U.S. Pat. No. 4,633,143 issued to McCartney on Dec. 30, 1986.
Nakayama et al discloses a means for focusing a color CRT by placing a first high voltage (HV) on the layer of the conical portion of the CRT. A second high voltage (HV) for operation of the electron gun is supplied via a lead wire extending within the conical portion of the CRT through a conductive tubular member. The conductive tubular member is electrically connected with the conductive layer by an arcuate spring member which further serves to accurately and securely position the tubular member and the lead wire therein within the tube envelope, thereby ensuring proper engagement with the gun of a contact member that extends from the lead wire.
Marioka et al describes a TV color camera tube having individual means for supplying current to the red, green, and blue focusing coils. A main focus current control circuit is connected in series with the series-connected focusing coils for controlling a main focus current so as to maintain an adjustably predetermined value thereof, and individual focus current control circuits respectively corresponding to the image pick-up tubes are connected in parallel with the respective focusing coils for controlling the individual focus current flowing through each of the focusing coils without influencing the individual focus currents through the other coils.
McCartney discloses a delta gun CRT wherein the focus position for each of the electron gun is pre-calculated to position the beam in the four quadrants of the CRT face. The quadrants of each of the gun's coordinate system is the same as the quadrants of the CRT screen thereby allowing each of the coefficients to be changed or adjusted only when the terms it multiplies is zero. Complete convergence correction for the entire screen may be accomplished using only nine screen locations.
Because of manufacturing tolerances it has hitherto been difficult to bring the three color electron guns into a high degree of resolution. The prior art taught that the focus supply voltage source was designed to control the focus bias potential on all three color electron guns. However, the prior art devices did not provide a means for individually controlling the focus bias potential of the three guns.